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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS Part 7 (Details) | 12 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2011
Pension Plans [Member] |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 3.95% | [1] | 5.10% | [1] | 4.75% | [1]
Weighted average rate of future compensation increase | 3.00% |  | 3.00% |  | 3.00% |
Expected long-term rate of return on plan assets | 7.75% |  | 7.75% |  | 8.00% |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.70% | [2] | 3.95% | [2] |  |
Weighted average rate of future compensation increase | 3.00% |  | 3.00% |  |  |
Pension Plans [Member] | Missouri Gas Energy (MGE) [Member] |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 5.05% |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 5.00% |  |  |  |  |
Postretirement Plans [Member] |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 3.80% | [3] | 5.05% | [3] | 4.70% | [3]
Weighted average rate of future compensation increase | 3.00% |  | 3.00% |  | 3.00% |
Expected long-term rate of return on plan assets | 7.75% | [4] | 7.75% | [4] | 8.00% | [4]
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.60% | [5] | 3.80% | [5] |  |
Weighted average rate of future compensation increase | 3.00% |  | 3.00% |  |  |
Postretirement Plans [Member] | Missouri Gas Energy (MGE) [Member] |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 5.05% | [3] |  |  |  |
Expected long-term rate of return on plan assets | 5.75% | [4] |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.95% | [3] |  |  |  |
[1] | Weighted average discount rate assumption for the MGE pension plan is 5.05%.
[2] | Derivative assets of $3,027 net of cash margin payable of $2,750.
[3] | Weighted average discount rate assumption for the MGE postretirement plan is 5.05%.
[4] | Expected long-term rate of return on plan assets assumption for the MGE postretirement plan is
[5] | Weighted average discount rate assumption for the MGE postretirement plan is 4.95%.

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- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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